First thing to check is make sure you're just not being dumb and have the fader on the head unit set all the way to the rear. If that is fine, on to some trouble shooting. First, set the front RCA output to the rear amplifier and see if there is sound. If there is, you know its something wrong from the amplifier to the speakers. If nothing is wrong there, try hooking up your head unit speaker outputs to the front speakers. If they work, then you know your speaker wires are fine.

Did you change anything in your system at all, other than the head unit?